Showing papers in "Journal of the American Chemical Society in 1990"

Semianalytical treatment of solvation for molecular mechanics and dynamics

Abstract: sorption results9 revealed that the iron surface was mostly covered by promoter oxides of AI, Ca, and K. Postreaction XPS results also revealed a C( Is) XPS peak of weak to moderate intensity centered at 284.1-283.7 eV. This binding energy approaches those (ca. 283.5 eV) reported for iron cat bide^.^^*'^ More convincing evidence for carbide formation was obtained from TPHT results collected after reaction studies like those displayed in Figure 1 in which methane was the only product. After reaction at temperatures below 340 OC, only small amounts of reactive carbon could be distinguished with maximum methane desorption rates near 300 OC. However, for higher reaction temperatures, large amounts of methane were produced with a maximum rate just above 400 OC. Since XPS results revealed only small amounts of carbonaceous residue on top of the catalyst surface, this reactive carbon must be associated with carbiding of the catalyst. Consequently, it appears that the active carbon incorporation catalyst is carbided iron. This conclusion is well supported by bulk carbon to iron stoichiometries of 0.1-0.25 estimated from the TPHT peak areas which were adequate to represent 40-60'36 conversion to bulk carbides such as Fe,C or FeSC2. Moreover, preliminary results from studies using bona fide iron carbides have shown similar catalytic b e h a ~ i o r . ~

Preparation of polymers with controlled molecular architecture. A new convergent approach to dendritic macromolecules

Abstract: The novel convergent growth approach to topological macromolecules based on dendritic fragments is described. The polyether dendritic fragments are prepared by starting from what will become the periphery of the molecule and progressing inward. In the first step, 2 mol of a benzylic bromide is condensed with the two phenolic groups of the monomer, 3,5-di-hydroxybenzyl alcohol, under phase-transfer conditions. After transformation of the benzylic alcohol functionality of the growing molecule into the corresponding bromide, the procedure is repeated with stepwise addition of the monomer followed again by activation of the benzylic site. After several generations of growth, the resulting dendritic wedges, in their benzylic bromide form, can be coupled to a polyfunctional core such as 1,1,1-tris(4{prime}-hydroxyphenyl)ethane to form the final hyperbranched macromolecule. Unique features of the convergent approach include the control over the nature and placement of the groups that are placed at the periphery of the molecule and the fact that each growth step only involves reaction at a single site of the growing macromolecule.

Fundamental studies of microscopic wetting on organic surfaces. 1. Formation and structural characterization of a self-consistent series of polyfunctional organic monolayers

Abstract: Monolayers of a series of terminally substituted alkyl thiols, X(CH 2 ) 15 SH (X=CH 3 , CH 2 OH, CO 2 H, CO 2 CH 3 , and CONH 2 ), have been prepared by adsorption from solution onto evaporated gold substrates. The structures habe been characterized by infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), ellipsometry, and temperature-programmed desorption (TPD). The IR data shows the monolayer films to be densely packed, crystalline-like structures with all-trans conformation alkyl chains exhibiting average tilt angles of the chain axis in a range of 28-40° from the surface normal and an approximate 55° twist of the chain axis away from a configuration with the CCC plane perpendicular to the surface plane

Enantioselective Epoxidation of Unfunctionalized Olefins Catalyzed by (salen)Manganese Complexes

Abstract: need to be sulfonated in order to produce the stable polysemiquinone form of the polymer. Indeed, additional sulfonation and consequent protonation of amine nitrogen atoms would convert some of the -(NH)to -(NHzf)groups and hence destabilize the polymer by reducing the extent of its T conjugation. The absorption maxima at 1080, 700, and 620 cm-' in the FTIR spectrum of compound IIIA,B are consistent with the presence9 of SO3groups attached to the aromatic rings. The absorption maxima at 820 and 870 cm-I indicative of 1,2,4-trisubstitution of the rings are out-of-plane bending of aromatic hydrogens. These absorptions are not present in the 1,2-disubstituted emeraldine base ( I I ) , from which compound IIIA,B was synthesized. The solubilities of compounds IIIA,B and IV differ markedly from those of the corresponding protonated and nonprotonated forms, respectively, of parent polyaniline. Compound IIIA,B dissolves appreciably in aqueous 0.1 M N H 4 0 H or NaOH to give the corresponding salts whereas emeraldine hydrochloride when treated in this manner is converted to the insoluble emeraldine base form (11). The anionic (SO,-) groups are presumably largely responsible for its solubility in water. Compound IIIA,B partly dissolves in DMSO, giving the dark green color of the protonated polyaniline, but is apparently deprotonated when it dissolves in N-methyl-2-pyrrolidinone (NMP), in which it has a blue-violet color, characteristic of compound IV. The high concentration of protons in the vicinity of the polymer backbone due to the presence of the attached SO3groups is not only responsible for the retention of doping of the polymer at the higher pH values, where the parent emeraldine base polymer (TI) is essentially a (nondoped) insulator, but is also consistent with the observed faster electrochemical redox reactions of compound IIIA,B in aqueous media.

Molecular light switch for DNA : Ru(bpy)2(dppz)2+

Abstract: Considerable research has focused on the development of nonradioactive probes for nucleic acids. Extensive photophysical studies indicate that Ru(phen){sub 3}{sup 2+} bound to double-helical DNA displays an increase in luminescence owing to intercalation; emission from the metal-to-ligand charge transfer (MLCT) excited state decays as a biexponential with one lifetime of 2 {mu}s attributed to the intercalative form and a second lifetime of 0.6 {mu}s (indistinguishable from the free species) assigned to the surface bound form. Here we report the application of a novel transition-metal complex as a true molecular light switch for DNA. This probe is Ru(bpy){sub 2}(dppz){sup 2+} (bpy = 2,2{prime}-bipyridine, dppz = dipyrido(3,2-a:2{prime},3{prime}-c)phenazine), which shows no photoluminescence in aqueous solution at ambient temperatures, but displays intense photoluminescence in the presence of double-helical DNA, to which the complex binds avidly.

Coadsorption of ferrocene-terminated and unsubstituted alkanethiols on gold: electroactive self-assembled monolayers

Abstract: Self-assembled monolayers provide an ideal system for disentangling the fundamental events in interfacial electron transfer. Coadsorption of ferrocene-terminated alkanethiols with unsubstituted n-alkanethiols on evaporated gold films yields stable, electroactive self-assembled monolayers. Monolayers containing low concentrations of alkanethiols linked to ferrocene by a polar ester group (FcCO{sub 2}(CH{sub 2}){sub n}SH, Fc = ({eta}{sup 5}-C{sub 5}H{sub 5})Fe({eta}{sup 5}-C{sub 5}H{sub 4})) show thermodynamically ideal surface electrochemistry in 1 M HClO{sub 4}, indicating the ferrocene groups to be homogeneous and noninteracting. Higher surface concentrations or use of alkanethiols linked directly to the nonpolar ferrocene group (Fc(CH{sub 2}){sub n}SH) lead to broadened electrochemical features, indicating interactions among ferrocene groups or inhomogeneous sites. Longer chain lengths and lower ferrocene surface concentrations result in slower electron-transfer kinetics with the ferrocene groups. A fraction of the thiols in a monolayer exchange with thiols in an ethanol solution, but much of the monolayer remains unequilibrated after 10 days.

Synthesis of molybdenum imido alkylidene complexes and some reactions involving acyclic olefins

Abstract: The reaction between Mo(C-t-Bu)(dme)Cl{sub 3} (dme = 1,2-dimethoxyethane) and Me{sub 3}SiNHAr (Ar = 2,6-diisopropylphenyl) yields Mo(C-t-Bu)(NHAr)Cl{sub 2}(dme) (1), which upon treatment with a catalytic amount of NEt{sub 3} is transformed into Mo(CH-t-Bu)(NAr)Cl{sub 2}(dme) (2). Complexes of the type Mo(CH-t-Bu)(NAr)(OR){sub 2} (OR = OCMe(CF{sub 3}){sub 2}, OCMe{sub 2}(CF{sub 3}), O-t-Bu, or OAr) have been prepared from 2. Complexes of the type Mo(C-t-Bu)(NHAr)(OR){sub 2} (OR = OCMe(CF{sub 3}){sub 2} or OAr) have been prepared from 1, but they cannot be transformed into Mo(CH-t-Bu)(NAr)(OR){sub 2} complexes. A precursor to imido alkylidene complexes that is related to 2 has been prepared by the sequence MoO{sub 2} {yields} MoO{sub 2}Cl{sub 2} {yields} Mo(NAr){sub 2}Cl{sub 2} {yields} Mo(NAr){sub 2}(CH{sub 2}R{prime}){sub 2} {yields} Mo(CHR{prime})(NAr)(OTf){sub 2}(dme) (R{prime} = t-Bu or CMe{sub 2}Ph; OTf = OSO{sub 2}CF{sub 3}). Mo(CH-t-Bu)(NAr)(OTf){sub 2}(dme) crystallizes in the space group P{anti 1} with a = 17.543 {angstrom}, b = 19.008 {angstrom}, c = 9.711 {angstrom}, {alpha} = 91.91{degree}, {beta} = 99.30{degree}, {gamma} = 87.27{degree}, Z = 4, M{sub r} = 729.60, V = 3,191.1 {angstrom}{sup 3}, {rho}(calcd) = 1.518 g cm{sup {minus}3}.

Nucleation and Growth of CdSe on ZnS Quantum Crystallite Seeds and Vice Versa, in Inverse Micelle Media

Abstract: Composite semiconductor crystallites involving CdSe grown on an ZnS seed, and vice versa, have been synthesized and capped with organic ligands in inverse micelle solutions. These composite particles, as well as capped seed crystallites of CdSe and ZnS, are isolated, purified, and characterized for relative atomic composition, structure, and electronic properties. The Debye X-ray scattering equation, when solved for these layered particles, shows that powder X-ray scattering is insensitive to a small foreign inclusion. A simple theoretical model for the LUMO and HOMO of layered crystallites shows that a small (< 15-{angstrom} diameter) interior foreign seed causes only small shifts of the lowest excited state, to either higher or lower energies. The capped CdSe seed and the capped CdSe portion of the layered particle grown on a ZnSe seed undergo low-temperature (169{degree}C) annealing to give near-single-crystal X-ray scattering. However, CdSe annealing is blocked by a surface ZnS layer which is ca. 4 {angstrom} thick. While growth to make composite particles does occur, neither particle shows evidence for epitaxial growth.

Chemical bonding in hypervalent molecules. The dominance of ionic bonding and negative hyperconjugation over d-orbital participation

Abstract: Does sulfur form six covalent bonds in CH{sub 3}SO{sub 2}Cl, F{sub 3}S{triple bond}N or carbon or phosphorus five bonds in F{sub 3}C=O{sup {minus}}, F{sub 3}P=O After a brief history of the viewpoints on hypervalent bonding and a comparison of analysis methods (with CH{sub 3}SO{sub 2}Cl as example), natural population and natural bond orbital analysis is applied to a series of 32-valence-electron species of X{sub 3}XY type (CF{sub 4}, F{sub 3}NO, O{sub 3}ClF, O{sub 3}PS{sup 3{minus}}, F{sub 3}SN, etc.). The {sigma}-bonding in these systems is found to be significantly ionic, and the strongly polar {sigma}*{sub AX} orbitals are found to be more effective electron acceptors than the extra-valence d{sub {pi}} (A) orbitals. By generalizing our discussion to n-coordinate 8n-valence-electron species (HF{sub 2}{sup {minus}}, BF{sub 3}, ClO{sub 4}{sup {minus}}, F{sub 4}SO, F{sub 5}TeO{sup {minus}}, IF{sub 6}{sup +}, OXeF{sub 6}, etc.) and their reduced analogues that have one or more lone pairs on the central atom (SF{sub 4}, IF{sub 6}{sup {minus}}, ClF{sub 3}, etc.), we provide a classification of hypervalent (and many nonhypervalent) molecules. The simple, qualitative bonding concepts for hypervalent molecules developed here supercede the inaccurate and misleading dsp{sup 3} and d{sup 2}sp{sup 3} models that are still in widespread use.

Probing conformational changes in proteins by mass spectrometry

Abstract: Mass spectrometry has found wide application for the elucidation of the primary structures of proteins. However, with the exception of topographical studies of membrane-bound proteins, mass spectrometry has not previously been utilized to obtain information concerning in three-dimensional conformation of proteins. In the present communication, the authors describe the first use of mass spectrometry for probing conformational changes in proteins in a manner analogous to that employed in techniques like optical rotary dispersion, circular dichroism, and spectrophotometry.

Synthesis of self-doped conducting polyaniline

Abstract: Caracterisation par spectrometrie UV, FTIM et RPE des polymeres obtenus par sulfonation de la polyaniline. Les polymeres sont solubles en solution aqueuse basique

Platinum-195 NMR kinetic and mechanistic studies of cis- and trans-diamminedichloroplatinum(II) binding to DNA

Abstract: The kinetics and mechanism of binding of the anticancer drug cis-diamminedichloroplatinum(II), or cis-DDP, and its inactive trans isomer to chicken erythrocyte DNA at 37 o C have been investigated by 195 Pt NMR spectroscopy. Both cis-and trans-DDP bind to DNA by two successive pseudo-first-order processes, forming monofunctional adducts ( 195 Pt NMR shifts near -2300 ppm) that subsequently close to bifunctional lesions (chemical shifts near -2450 ppm)

Structures and Configurations of Ciguatoxin from the Moray Eel Gymnothorax javanicus and Its Likely Precursor from the Dinoflagellate Gambierdiscus toxicus

Abstract: Ciguatoxin (CTX) is the toxic principle of ciguatera, which is responsible for the most widespread food poisoning of nonbacterial origin. The toxin, isolated from the moray eel Gymnothorax javanicus, and its congener, from the causative dinoflagellate Cambierdiscus toxicus, were used for this study. The structure elucidation was carried out by combined use of 'H NMR 2D correlation and NOE experiments done with no more than 0.35 mg of CTX and 0.74 mg of the congener. Broadening of 'H NMR signals due to a slow conformational change around a nine-membered ring was sharpened by measurements at -20 OC, in which all 'J proton connectivities and NOES around angular protons were clearly indicated. The structure of CTX, which had a molecular formula of CSOHMOI9, was disclosed to be a brevetoxin-type polyether comprising 13 continuous ether rings (7/6/6/7/7/9/7/6/8/6/7/6-spiro-5). The congener was shown to be a less oxygenated analogue of CTX. Their relative stereochemistries, except for C2 of CTX, were clarified by detailed analyses of IH NMR NOE experiments, MM2 energy calculations, and spectral simulations. Ciguatera is a term applied to food poisoning caused by in- gestion of coral reef fish. The worldwide occurrence of ciguatera not only endangers public health but also hampers local fisheries in subtropical and tropical regions. It is estimated that roughly 20000 people suffer annually from the poisoning, making it one of the largest-scale food poisonings of non-bacterial origins. The toxification mechanism of fish had not been known until one of the authors (T.Y.) identified an epiphytic dinoflagellate, Gambierdiscus toxicus, as a causative organism in 1977.' The

Synthesis and Characterization of Surface-Capped, Size-Quantized CdS Clusters. Chemical Control of Cluster Size

Abstract: Clusters of CdS in the quantum confinement regime C5O-A diameter are prepared in a rational technique whereby the cluster size and its distribution are controlled by chemical means. Competitive reaction chemistry between CdS core cluster growth and surface capping by thiophenolate leads to clusters whose core is essentially sphalerite CdS but whose reactive surface has been passivated by covalently attached phenyl groups. Adjustment of the sulfide to thiophenol ratio during synthesis takes advantage of the competitive reaction rates of these species with Cd ions to control the eventual cluster size. The clusters remain soluble in several organic solvents but may be isolated as stable powders and subsequently redissolved. The Cd "'NMR data for this series of capped clusters confirm the presence of sphalerite CdS as the cluster core and the increasing percentage of Cd involved in this core as the S/SPh ratio increases. Optical properties demonstrate well-behaved absorption edge and emission band shifts with development of exciton features as the clusters grow. Colloidal semiconductor species are currently under intense investigation as examples of nonmolecular materials that dem- onstrate the effects of quantum confinement.' The enhanced photoreactivity and photocatalysis as well as the predicted effects on nonlinear optical properties of these species has led to a wide range of synthetic approaches to these materials.* The key to any synthetic investigation of this sort must be the careful control of semiconductor cluster size and, even more important, the control of the size distribution. The relatively amorphous character of small clusters prepared in this way necessitates the use of structural probes that provide information even in the absence of long-range periodicity. While X-ra diffraction can offer information for particles in excess of 25 in crystalline dimensions, smaller particles are X-ray amorphous and larger sizes (>lo0 A) can suffer from significant contributions to the line widths by strain broadening. Quantitative interpretation using X-ray diffraction alone is therefore difficult. Among the various techniques suitable for such problems, solid- state NMR has the distinct advantage of providing element-se- lective, inherently quantitative information about local coordination environments and symmetries. In addition, NMR chemical shifts are also expected to be sensitive to cluster sizes, since the size quantization effects present in small semiconductor clusters should lead to an increase in the average excitation energy parameter in the paramagnetic term of Ramsey's chemical shift theory. This has been recently verified by Duncan and co-workers in a liq- uid-state "Se NMR study of colloidal CdSe sol~tion.~ We wish to describe a new synthetic approach to such colloids of CdS based on the competitive growth/termination of CdS species in the presence of thiophenol surface-capping agents. The reports by Steigerwald et aL4 using a micellar approach to ben- zeneselenol-capped CdSe clusters and by Dance et aLs on the preparation of a molecular fragment of sphalerite CdS where a Cdl& core was capped by 16 SPh groups led us to investigate this approach to the rational control of CdS cluster size by capping of the cluster surface by thiophenol-like species. Systematic, detailed optical and NMR behaviors have been revealed. Our NMR data, in addition, complement previous wide-line NMR studies undertaken on bulk cadmium sulfideb6 and add to the extremely limited database presently available for non-oxide chalcogenide systems.